1. Technical Field
The present invention relates generally to a short arc type mercury lamp and, more particularly, to a short arc type mercury lamp in which Hg and a rare gas are enclosed inside a light-emitting tube.
2. Description of the Related Art
Typically, short arc type mercury lamps in which Hg and a rare gas are enclosed are used as a light source for exposing semiconductor, a liquid crystal display (LCD) or the like to light.
Japanese Laid-Open Patent Publication 2003-234083 (Patent Document 1) discloses examples of the short arc type mercury lamp each of which is configured by enclosing each of Ar, Kr and Xe therein as a rare gas.
According to this document, in the lamps in which the rare gases are enclosed under the same pressure, when the lamps are lighted under the same conditions, it is suggested that the lamp in which Ar is enclosed can acquire the highest intensity of radiation from emitted light.
Describing in brief, the reason why the intensity of radiation varies depending on the sort of the rare gases is caused by the different thermal conductivities of the gases. Since the mercury arc can contract at a higher thermal conductivity, the arc is elongated, thereby increasing the current density. This can consequently realize a light source having higher luminance. The sequence of heat conductivity is Ar>Kr>Xe, and the intensity of radiation on an irradiated surface increases in this sequence.
Referring to the schematic view of the arc in FIG. 7A and FIG. 7B, the size of an arc of a lamp A in which Ar gas is enclosed is compared with the size of an arc of a lamp B in which Kr gas is enclosed. In the lamp A in which Ar gas is enclosed, between a cathode and an anode, the arc slightly spreads in the direction toward the anode but its width (diameter) is restricted. Accordingly, the arc is contracted, more particularly, contracted about the leading end of the anode. In contrast, in the lamp B in which the Kr is enclosed, the arc that extends from the leading end surface of the cathode is continuously spread toward the anode. Accordingly, almost the entire area of the leading end surface of the anode, i.e. a wider area, is subjected to the arc.
In this point of view, it is typical that Ar is enclosed in the short arc type mercury lamp in the related art in order to realize high luminance.
However, in the short arc type mercury lamp in which Ar gas is enclosed, in particular, in the lamp in which Ar is enclosed at a positive pressure of 0.25 MPa or greater, the persistency ratio of intensity of radiation may suddenly decrease in some cases when a light-on time exceeds a predetermined time, for example, 1500 hours.
When the inventors inspected the reason for the sudden decrease in the intensity of radiation of the lamp, it became clear that no problem occurs when the lamp is lighted on at a lamp current of 150 A or less but the decrease in the intensity of radiation became significant when the lamp current was 180 A or greater.
In the lamp in which the persistency ratio of the intensity of radiation is decreased, as shown in FIG. 8, concaves and convexes X formed on the leading end surface of the anode were observed. The reason for this is considered to be that, since the arc is contracted by the Ar gas, the arc is locally concentrated at the leading end surface of the anode, such that the current density is increased and the leading end is overheated, thereby generating thermal stress, which then deforms the leading end surface of the anode.
When the leading end of the anode is deformed, the arc is concentrated on the deformed portion, thereby intensifying overheating. The intensifying overheating evaporates tungsten, or the anode material, which is then deposited on the light-emitting tube, thereby causing blackening to progress. It is regarded that this series of phenomenon suddenly progresses after the passage of a certain time, thereby suddenly lowering the intensity of radiation.
This phenomenon is not observed in any lamp in which Kr or Xe is enclosed instead of Ar.
In other words, when it is intended to increase simply the longevity of the lamp, it is possible to overcome this problem by using a rare gas, for example, Kr gas, that has lower thermal conductivity than Ar.
However, in this case, as described above, it is impossible to contract the arc in the elongated shape as in the case of using Ar, thereby failing to achieve a high intensity of radiation on the irradiated surface. This leads to another problem in that the initial intensity of radiation as required cannot be achieved.